Interpretive Summary: Listeria monocytogenes is a zoonotic bacteria that has been frequently associated with dairy production. Although pasteurization effectively kills this organism, it is still important to minimize the contamination of raw milk with Listeria monocytogenes. The objective of this study was to explore potential risk factors associated with the presence of viable Listeria monocytogenes in raw milk (BTM) from US farms. Our results suggest that herds with less than 100 milking cows and herds located in the west are at a lower risk of having BTM contamination with L. monocytogenes than are larger dairies and/or dairies located in the southeast. Dairy production practices are very diverse in the US and further research needs to be conducted to identify risk factors for BTM contamination with L. monocytogenes at a national level.

Technical Abstract:
The objective of this study was to investigate herd characteristics and management practices as potential risk factors associated with the presence of Listeria monocytogenes in bulk tank milk (BTM) from US dairy farms. General management data and BTM samples were collected as part of the National Animal Health Monitoring System (NAHMS) Dairy 2002 survey. A total of 71 factors grouped into categories of herd characteristics, general management, feed types, milking procedures, manure handling and waste treatment, and health management were univariately evaluated for association with the presence of L. monocytogenes in BTM. The univariate analysis indicated that using automatic take offs and having an open herd management is associated with an increased risk of BTM contamination with L. monocytogenes, while storing manure in outside pens not accessible to cattle is associated with a lower risk of BTM contamination. These variables, however, were not sustained in the multivariable model. The multivariable model indicated that the presence of L. monocytogenes in BTM is significantly associated with region of the US and herd size. Farms located in the west are 6 times less likely to have BTM contamination with L. monocytogenes than are farms located in the southeast. A negative association was found between herd size and the presence of L. monocytogenes in milk. Compared to farms with more than 500 head, farms having less than 100 cattle are 5 times less likely to have L. monocytogenes in BTM. Our results raise the question of whether or not certain milking procedures (i.e. predipping, forestripping, etc.) previously found to be associated with L. monocytogenes in BTM in specific geographic areas have the same impact on milk contamination in the general US dairy population. Further research might be needed in order to evaluate risk factors for BTM contamination with L. monocytogenes that could be valid for most US dairies.